In an audio system, before audio signals are converted to the sound being played by loudspeakers, the audio signals are usually processed in different ways, such as undergoing amplifying, equalizing, crossover and other signal processing. This processing can be done in several discrete components, such as an equalizer, a digital signal processor (DSP), and an amplifier, or, alternatively, by an amplifier with equalization and digital signal processing functionality built into it.
For a typical factory sound system in a vehicle, the processing of audio signals is done similarly. However, since the vehicle sound system is responsible for playing not only the sound from the radio but also warning messages and chime sound from the vehicle, the factory amplifier is connected to and controlled by the vehicle data system. The factory amplifier often receives other audio signals such as notifications from a factory GPS navigation system, voice audio from a Bluetooth hands-free phone system, or an external audio device such as a remote mounted CD player or USB media player. In many cases, the amplifier also adjusts the system volume and controls the fader and balance based on data messages sent on the vehicle data network by the factory radio.
Many vehicle owners desire to improve the sound or functionality of the factory audio system. This usually requires replacement of their factory sound components with aftermarket sound components such as amplifiers with more power and higher fidelity, and loudspeakers with more power handling capability and better sound quality. However, in vehicles with amplifiers connected to vehicle data networks, replacing the factory amplifier and the factory loudspeakers is not as simple as removing those components, installing the new ones and reconnecting the wires. This is because the factory amplifier is specifically designed to be more functional and is controlled by a set of data messages sent on the vehicle data network. These messages vary based on the vehicle make, model and audio system. Aftermarket amplifiers are not configured to work with any of the vehicle data networks.
Typically, to work around this issue, the factory amplifier is not replaced when an aftermarket amplifier and speakers are added. Instead, an aftermarket amplifier is added to the system after the factory amplifier and a digital signal processor unit is installed between the factory amplifier and the aftermarket amplifier. Since the outputs of the factory amplifier are band limited and designed to drive the particular factory speakers, it is often necessary to sum several output channels from the factory amplifier to get a full-range input for the aftermarket amplifier from 20 to 20 KHz. The digital signal processor converts the analog audio signals to digital signals and combines several output channels. Then, it is used to invert any equalization and time alignment done by the factory DSP amplifier, and outputs a full spectrum (20 Hz-20 KHz) flat (with no frequency boosted or attenuated) to the aftermarket amplifier to re-process the signals in a way that fits the aftermarket loudspeakers. Once a flat signal is obtained, the DSP unit may re-process the signal for the new system prior to transmitting it to the aftermarket amplifier. Typical re-processing includes providing boost or attenuation of specific frequencies, splitting the signal up for different speakers, and time aligning the signal to perfect the stereo image.
This typical signal re-processing has significant drawbacks. One drawback is that the sound quality is often affected because the audio signals are processed, reverted and processed again before they are played by the loudspeakers. Second, it is often very complex and difficult to reconstruct audio signals that have already been processed by an amplifier. As such, a good DSP unit is often very expensive and is hard to set up properly. Third, such a system often results in warning chimes, voice prompts and Bluetooth telephone audio being louder than they should be or being played in all of the new loudspeakers rather than in specific loudspeakers such as the left front speaker. Fourth, sometimes the factory amplifier processes the audio signals in such a way that it is irreversible, thus preventing the aftermarket sound systems from perfectly reconstructing the ideal audio signals and playing the best quality of sound.
In the alternative, some installers opt to replace the factory amplifier with an aftermarket amplifier to gain access to the full-frequency, flat audio signals provided by the radio to the factory DSP amplifier. However, removing the factory amplifier will deprive the vehicle of much functionality that is handled by the vehicle's data system. Since the factory amplifier is configured to control the main system volume, connecting an aftermarket amplifier to the radio output directly will result in the loss of system volume control. Also, the warning chimes and voice notification from the vehicle will also be lost. In addition, in most cases this will result in the inability to fade the sound from front to rear or balance the sound from left to right in the new system.